Metal detector method for identifying target size

ABSTRACT

A metal detector utilizes digital signal processing and a microprocessor to process buffers of information which is received at a periodic ram. Both high and low gain phase quadrature and in-phase signals are provided via a multiplexer to an analog-to-digital converter from a first receive antenna. A second receive antenna provides phase quadrature and in-phase signals also through the multiplexer to the analog-to-digital converter. The received signals are averaged, decimated and low pass filtered to eliminate noise and reduce the quantity of data which must be processed. A threshold (triggering) processing operation is performed to determine whether a valid target signal is present in the dam. If not, no further processing is performed. The in-phase and quadrature components are processed using Fourier transforms to select a frequency band which includes the energy for the target signal. The energy in this frequency band is utilized to determine the identification of the target. The depth of the target is determined by comparing the quadrature phase components received from the first and second receive antennas. The size of the target is determined by reference to a look-up table based on the depth factor and the signal amplitude determined for the target object. A display screen has a plurality of horizontal depth symbols, each of which has a plurality of size indicators and upon determining the depth and size of a target, one depth symbol is activated together with one of the size indicators to concurrently display this information to an operator.

TECHNICAL FIELD OF THE INVENTION

The present invention pertains in general to metal detectors, and inparticular to metal detectors which measure and indicate characteristicsof the target.

BACKGROUND OF THE INVENTION

It has been well established in the metal detector industry that targetscan be identified based upon their physical composition due to phasecharacteristics of receive signals produced when the target object is inthe field of the transmit and receive coils. It has further beenestablished that the effect of mineralized soil, which can produce aground component signal, can be minimized by processing techniques whichrecognize the different frequency response characteristics of a targetand the ground. Dual receiver antenna designs have also been suggestedas a possible technique for determining distance to a target. Despitethese advances, however, there exists a need for better methods ofprocessing data to extract information about the target signal and todetermine the actual size of the target which has been detected. Thepresent invention provides for improved identification of the physicalcomposition of the target, reduced incidence of false alarms, the depthof the target and concurrent identification of the size of the target.

SUMMARY OF THE INVENTION

A selected aspect of the present invention comprises a method fordetecting the presence of a target signal within a received signal of ametal detector. A Fourier component of a quadrature phase target signalis examined with certain requirements to determine the existence of atarget signal. The portion of the Fourier signal which meets theserequirements establishes a frequency range that represents the energy ofthe target signal. The same frequency range is examined for the in-phasesignal component to select the target information from that phasecomponent. These signal components are ratioed to determine theelectrical or conductive characteristics (ID) of the target.

In a further aspect of the present invention, a preliminary examinationof signals, in the generation of a new vector, is used to determine ifthere is a likelihood that the presently received, and buffered, signaldata includes a target signal. The new vector comprises a combination ofthe in-phase and quadrature phase components of a first receive coilsignal. Both the quadrature phase component of the receive signal andthe new vector signal are processed through a high pass window filterand these signals are multiplied to get a product. This product iscompared to a threshold value as well as polarity requirements todetermine if a target signal is present and further processing of thedata sorted in the buffer should be performed.

In a still further aspect of the present invention, the size of a targetobject is determined after the identification of the target, the depthand the amplitude of the target signal has been determined byprocessing. These factors are entered into a look-up table whichidentifies a size classification for the object.

In a still further aspect of the present invention, a unique display fora metal detector indicates concurrently the size and depth of a target.A plurality of depth symbols are arrayed vertically with horizontalelongated elements. Each of the depth symbols includes a plurality ofsize indicators. When a target is determined to exist, a correspondingdepth symbol is activated (illuminated) and one of the size indicatorswithin the depth symbol is also illuminated so that the operator ispresented with information in a graphic form showing the size and depthof the target.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now taken to the following descriptiontaken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a hand-held metal detector whichincorporates the features of the present invention,

FIG. 2 is a display screen where the detector shown in FIG. 1 with thescreen illustrating the approximate size and depth of target objects,

FIG. 3 is an illustration of the single transmit coil and two receivecoils of the detector shown in FIG. 1,

FIG. 4 is a block diagram of the electronic circuits in the detectorshown in FIG. 1,

FIGS. 5A and 5B represent the vector signals produced by the RX1 and RX2receive coils for the detector shown in FIG. 1, together with the vectorresponses for selected target objects,

FIG. 6 is a table which determines a classification size for a targetdetected by a metal detector based on the determined depth of the targetand the amplitude of the target signal,

FIG. 7 is a further illustration of the S1 signal produced by the RX1coil with a pulse produced by a target,

FIG. 8 is an illustration of a signal S1' which is produced by use of ahigh pass window filter for the S1 signal shown in FIG. 7,

FIG. 9 is an illustration of the wave form which comprises the productof the S1' and S2' signals and is compared to a threshold for producinga trigger state,

FIG. 10 is an illustration of the S1 signal which is received by the RX1receive coil and includes a pulse produced by a target,

FIG. 11 is an illustration of a target signal, such as S1, which hasbeen stored in a 64 point buffer,

FIG. 12 is an illustration of the signal shown in FIG. 11 with thecenter of the target pulse aligned at the center of an FFT buffer,

FIG. 13 is an illustration of the magnitude of Fourier transform for thesignal shown in FIG. 12,

FIG. 14 is an illustration of the real component for the Fouriertransform signal shown in FIG. 13,

FIG. 15 is an illustration of a sample real component of a Fouriertransform for the S1 signal for another sweep of the detector,

FIG. 16 is a further example of the real component of the Fouriertransform for the S1 received signal showing the target energy within adefined window,

FIGS. 17A-17F comprise a flow diagram describing the detailed operationof the digital signal processor (DSP) shown in FIG. 4 and whichcomprises a component of the electronics of the metal detector describedin the present application, and

FIGS. 18A-18B comprise a flow diagram describing the operation of themicroprocessor shown in FIG. 4 and which is included within the metaldetector described herein.

DETAILED DESCRIPTION

Referring to FIG. 1, there is illustrated a hand-held metal detector 30which is typically used by hobbyists in searching for metal objectswhich are generally out of sight and typically buffed a short distancebeneath the surface of the earth. The detector 30 includes a stemsection 32, a hand grip 36 and a housing 38 which includes electronicdetection circuitry, further described below, together with a display 42and an operator touch pad 44. The detector 30 further includes a searchhead 44 which includes transmit and receive coils that are connected viaa cable 46 to the electronics package 38. The cable can extend throughthe stem 32. The detector 30 includes an arm rest 48 and a battery pack50.

A detector of the general type shown in FIG. 1 is described in U.S. Pat.No. 5,148,151 entitled "Metal Detector Having Target CharacterizationClassification" which issued on Sep. 15, 1992. This patent isincorporated herein by reference.

Referring to FIG. 2, there is shown the display 42 and touch pad 44,which comprises an integral unit. The display 42 includes a graphicdisplay for indicating the depth and the size of a target object whichhas been detected by the metal detector 30. The depth of the target isindicated by illumination of one of the six vertically stacked depthsymbols which are identified as symbols 54, 56, 58, 60, 62 and 64. Theshallowest depth symbol is 54 and the deepest depth symbol is 64. In atypical scaling, the symbols 54 through 62 indicate two-inch intervalsand the symbol 64 indicates a depth of twelve inches or greater.

Within each of the depth symbols 54-64, there are size indicators acrossthe top scale as A, B, C, D and E. Each of the size indicators is anellipse and all of the ellipses have a common center. The activation ofindicator A shows the smallest size target while the activation ofindicator E shows the largest size target. The size scale is set forthas follows:

Size A--Smaller than a coin.

Size B--Coin size.

Size C--Larger than coins up to small belt buckles, such as pull-tabsand bottle caps.

Size D--Small belt buckles up to aluminum drink cans.

Size E--Anything bigger than an aluminum can, such as a kettle.

The touch pad 44 shown in FIG. 2 provides operator control for the metaldetector 30. This includes power on/off, target characterization foraccepting and rejecting and a menu of selectable items for operation.

Target Talk is selected for speech on demand for the last registeredtarget. Non-Motion is an operating mode which does not require motion,that is, for static operation. Fast Track is an automatic groundbalancing operation. Treasure Image, press and hold, is for pinpointdetection. This gives true size and depth, with no motion. Motion is themost typically used mode for the features described herein. Last Modereverts the detector to the previously selected mode.

In accordance with the present invention, when the metal detector 30 hasdetected a target, there will be produced on the display 42 anillumination of one of the size ellipses A through E on one of the depthsymbols so that the operator is provided with an indication of the depthof the detected target as well as the approximate size of the target.This information is determined by the processing of the detected signalsfrom the target as described herein.

A further metal detector patent using signal processing fordiscrimination of targets is U.S. Pat. No. 4,700,139 entitled "MetalDetector Circuit Having Selectable Exclusion Range for Unwanted Objects"which was issued on Oct. 13, 1987. The patent is incorporated herein byreference. A still further patent which discloses digital signalprocessing for a metal detector is U.S. Pat. No. 4,709,213 entitled"Metal Detector Having Digital Signal processing" which was issued onNov. 24, 1987. This patent is incorporated herein by reference.

The search coil 44, shown in FIG. 1, is illustrated in further detailwith the coil configuration shown in FIG. 3. The search head 44 includesa transmit coil 92 and two receive coils 94 and 96 which arerespectively identified as the RX1 coil and the RX2 coil. The receivecoils RX1 and RX2 are offset from each other. In a preferred embodiment,the receive coils 94 and 96 are coaxial. The transmit coil 92 is coaxialwith the receive coils 94 and 96 and may be either coplanar with thereceive coils or offset somewhat. Each of the receive coils isinductively balanced with the transmit coil. As a result, when thetransmit coil receives a steady sinusoidal signal, there is essentiallyno output signal produced from the receive coils 94 and 96 when there isno target in the vicinity of the coils. However, when a conductive orinductive target or mineralized soil is in the vicinity of the coils 92,94 and 96, the coil coupling is unbalanced and a signal is produced inone or more of the receive coils 94 and 96 in a manner that is wellknown in the industry. In a selected embodiment, the transmit coil has adiameter of approximately 8.5 inches, the receive coil 94 (RX1) has adiameter of approximately 5.5 inches and the receive coil 96 (RX2) has adiameter of approximately 2.5 inches.

A block diagram of the electronics for the metal detector 30, shown inFIG. 1, is illustrated in FIG. 4. The RX1 receive coil is connected toan RX1 detector circuit and the RX2 receive coil 96 is connected to anRX2 detector circuit 102. The circuits 100 and 102 are phase detectorcircuits such as shown in U.S. Pat. No. 4,700,139 and U.S. Pat. No.4,709,213 which have been incorporated herein by reference.

The detector circuit 100 produces four different signals which aretransmitted through a multi-conductor line 104 to a multiplexer 106. Thedetector circuit 102 produces two signals which are transmitted througha multi-conductor line 108 to the multiplexer 106. The terminology forthe signals produced by each of the circuits 100 and 102 is listed asfollows:

S1L--RX1 coil phase quadrature signal low gain

S1H--RX1 coil phase quadrature signal high gain

S4L--RX1 coil in-phase signal low gain

S4H--RX1 coil in-phase signal high gain

S5--RX2 coil phase quadrature signal

S6--RX2 coil in-phase signal

The multiplexer 106 cycles through the six signals from the circuits 100and 102 and provides each one sequentially through a line 110 to ananalog-to-digital converter 112. The six analog signals noted above areconverted to digital words, preferably having 16 bits, and sequentiallyare provided through a digital bus 114 to a digital signal processor116. A selected embodiment of the digital signal processor 116 is amodel ADSP-2103 manufactured by Analog Devices. The operation of thedigital signal processor 116 is described in detail in reference toFIGS. 17A-17F below.

The digital signal processor 116 is connected through a bidirectionalbus 118 to a microprocessor 120. The operation of the microprocessor 120is described below in reference to FIGS. 18A-18B. A selected embodimentof the microprocessor 120 is a model 68HC705C8 manufactured by Motorola.

The microprocessor 120 drives an audio circuit 122 for producingselected audio responses when targets are detected or other conditionsoccur in the metal detector 30. The microprocessor 120 is connected tothe audio circuit 122 via a line 124.

The microprocessor 120 is connected via a bidirectional bus 126 to thekeypad 44 for providing operator input into the metal detector 30.

The microprocessor 120 is connected through a line 130 to the liquidcrystal display 42, which is shown in FIG. 2.

Referring now to FIG. 5A, there are shown two of the signals produced bythe receive coil RX1. These are S1H and S4H. The low gain signals S1Land S4L are located at the same angular positions. The vector diagramsshown in FIGS. 5A and 5B illustrate responses from targets and otherobjects in-phase relation to the transmitted signal. The S4H signal isapproximately in-phase with the transmitted signal and the S1H is inapproximate quadrature phase with the transmitted phase. As can be seen,ferrous soil produces a response which is very close to the in-phasesignal. Other targets have gradually increasing quadrature phasecomponents as shown by bottle cap, nickels, pull-tab, rings and coins.In a preferred embodiment, the S1H signal may be almost exactly in-phasequadrature or may be slightly offset as shown. Likewise, the S4H signalmay be almost exactly in-phase or slightly offset as shown.

Referring to FIG. 5B, the signals from coil RX2 are shown as vectors.The in-phase component S6 is slightly offset from being exactly in-phasewith the transmitted signal and the quadrature phase component S5 isshown as being slightly offset from a quadrature component, althougheach of these signals may be nearly exactly the phase of thecorresponding in-phase or quadrature component.

The S1H signal has been amplified to a higher level than the S1L signalbut has the same phase characteristic. This is true for the S4H and S4Lsignals as well. This provides for a greater dynamic range. In general,when the high gain signal (H) is in saturation, the low signal will beused. However, if the high signal is not in saturation, it will be used.

A table 150 which is utilized by the microprocessor 120, shown in FIG.4, for determining the approximate size of a detected target is shown inFIG. 6. There are two input values to this table. These are targetdepth, which is shown at the left side of the table, and a target signalamplitude value which are the entries shown in the matrix cells of thetable 150. The output from the table 150 is a size indication which isshown as sizes A, B, C, D and E along the top. These are the same sizeindicators that are shown in display 42 in FIG. 2. The numbers insideeach cell represent approximately the normalized lower boundary of theamplitude for each size. The numbers are normalized, approximaterelative values.

Referring to FIG. 4, the analog-to-digital converter 112 produces a setof the six signals S1L, S1H, S4L, S4H, S5 and S6 5,000 times per secondand these are decimated to 128 per second by the DSP 116. A buffer ofapproximately one-half second of data is maintained and processed. Eachtime a new set of samples is received, the oldest samples in the bufferare deleted and the newest set of samples are added to create a newbuffer of information for processing.

In a brief overview, the signal processing carded out by the metaldetector 30 digital signal processor and microprocessor is as follows.When each new set of data is received, that is, 128 times per second,the data in the buffer is examined to determine if it meets certainrequirements that indicate the presence of a target signal. If theserequirements are not met, the data is not processed and no responsesignals are produced. New data is awaited to update the information inthe buffer. If the preliminary examination of the data in the bufferindicates the presence of a target signal, then a sequence of operationsis carded out to process the data for determining the approximate sizeand approximate depth of the target. A Fourier transform operation isused to determine if a target signal is present in the S1 signal. Ifsuch a signal is present, the frequency band of the signal isdetermined. The same frequency band is examined for a Fourier transformof the S4 signal and the energy in this band is extracted. A ratio ofthese signals is produced to determine the electrical or conductivecharacteristic of the target, and this is referred to as a target ID. Inaddition, the S1 and S5 signal strengths are determined and compared asa ratio to determine target depth. The determined target depth isselected as the first factor for the table 150 and the determined targetsignal amplitude is entered as the second factor for the table. By entryof these two factors, a resulting approximate size indication from A-Eis determined. The depth and size indications are then sent to theliquid crystal display 42 which activates the appropriate depth symboland size indicator on that depth symbol to indicate to the operator theapproximate depth and size of the target object. This processing isdescribed in detail below.

A significant feature of the present invention is termed "adaptiveprocessing." It is well known that mineralized soil produces a signalcomponent that is approximate in-phase with the transmitted signal. Adesired target typically has a quadrature phase component andmineralized soil typically does not produce such a component. However,in order to determine the electrical characteristic of the target foridentification purposes, it is necessary to know both the quadrature andthe in-phase components of the target signal. A metal detector, such asdetector 30 shown in FIG. 1, is used by the operator by swinging it backand forth with the detector head 44 close to the surface of the ground.Thus, there is relative motion between the search head 44 and a target.Mineralized soil is generally uniformly distributed over the searcharea. Thus, the target signal will have a higher frequency response thanthe undesired mineralized soil signal.

A target pulse on the quadrature channel S1 is shown in FIG. 7. Thetarget pulse is represented by the reference numeral 151. As notedabove, the metal detector 30 samples at a rate of 128 hertz and storesapproximately 60 sets of samples which cover a time period ofapproximately one-half second. Thus, there is a "window" of data storedfor processing at any one time. As shown in FIG. 7, a window 152 of timeis indicated by the bidirectional arrows. Each 128ths of a second, thewindow 152 progresses one data sample set forward in time and deletesthe oldest sample set. As shown in FIG. 7, the window 152 has received asubstantial portion of the target signal pulse 151. The signal S1, aswell as the other five signals have all been digitized and stored in abuffer for the period of the time window 152.

When each buffer of data has been collected, the first step is to make athreshold determination of whether or not a target signal is present.This is now described in reference to FIGS. 7, 8, 9 and flow diagramFIGS. 17A and 17B. This is a "trigger" determination.

Referring to FIG. 17A, the digital signal processor 116 begins operationat the start bubble by entering operational block 180. Within thisblock, the digital signal processor reads the user interface informationthat is supplied to it from the microprocessor and has been entered bythe user into the microprocessor. This information establishesparameters for target searching. From block 180, the operation transfersto operational block 182 for reading the received coil signals providedby the analog-to-digital converter 112. Control is then transferred tooperational block 184.

Within block 184, the digital signal processor 116 averages a group ofcontinuous samples to produce an average sample value. Each of the sixsignals noted above is sampled at a rate of approximately fivekilohertz. Approximately forty samples are averaged to produce a singleresulting sample and these resulting samples occur at a rate of 128hertz. The resulting averaged signal is low pass filtered to eliminatehigh frequency noise components. The cutoff for high frequencycomponents is approximately 64 hertz.

From block 184, control is transferred to block 186 in which theprocessor 116 creates a seventh vector, which is termed S2. It is thesum of portions of the S1H and S4L vector signals. This summation isdone on a point-by-point basis. Control is then transferred tooperational block 188.

Within block 188, a window filtering operation is performed on the S1Hsignal and on the S2 signal to produce filtered signals which are termedS1' and S2'. This is shown in reference to FIGS. 7 and 8. The S1 signalin FIG. 7 has a target pulse with an amplitude of Δ. The windowfiltering performs a type of high pass filtering that preserves the fullamplitude of the target signal. Each signal value has subtracted from itthe signal value that occurred ten samples earlier. As noted above, eachsample occurs 128 times per second. As shown in FIG. 7, the signal valueis zero until the signal pulse is encountered. Thus, when the oldersignal is subtracted from the newer signal, the result is the productionof a signal S1' which has essentially the same shape as the originalsignal pulse. This is shown in FIG. 8. The result of this windowfiltering is the elimination of the DC component of the signal. As shownin FIG. 8 there will be some rebound as a result of this filtering, butthis can be ignored in the signal processing. The S2 signal, notedabove, is similarly processed using a high pass window filter to producea signal termed S2'. Control is then transferred to operational block190.

Within block 190, the signals S1' and S2' are multiplied by each otherto produce a result which is shown in FIG. 9. Control is thentransferred to question block 192 in FIG. 17B.

In question block 192, a flag is checked to determine if a target haspreviously been detected. If this flag has not been set, control istransferred to question block 198. If the flag is set, control istransferred directly to operational block 198.

In question block 194, a comparison is made to determine if the productsignal of S1' and S2' exceeds a preset threshold, this is shown in FIG.9. A further inquiry is made to determine if both the SI'and S2'signalsare negative. Both of these signals must be negative in order to have avalid target.

The signals shown in the various wave forms are expressed as positivesignals for ease of understanding but in an actual implantation asdescribed will be negative.

If the S1' and S2' signals meet both of the requirements set forth inquestion block 194, the yes exit is taken to a question block 196.Within block 196, a determination is made as to whether a local peak hasbeen reached for the S1 signal. The peak is determined when one samplehas two preceding values and two succeeding values that are both less inamplitude. This is five point peak detection. This inquiry is made todetermine if a sufficient amount of the target signal has been acquiredto initiate the processing of the target signals. If this requirement isalso met, control is transferred to the operational block 198.

If the requirements of either block 194 or block 196 are not met, the noexits are taken and control is returned to the start in FIG. 17A.

The transfer to the yes exit of block 196 is a confirmation that aviable target signal has been detected and that processing should beinitiated to determine the size and depth of the target. As noted, ifthese requirements for such a designation are not met, the processing isnot carded out for determining size and depth of the target. Byperforming this preliminary analysis and establishing a "trigger"requirement, it is less likely that the ultimate processing willinadvertently result in generating a target signal that could likely bebased on noise or other unintended signals. Thus, this increases thereliability of the detection signals produced by the metal detector 30to the operator.

Once the threshold triggering inquiry has been made and it has beendetermined by the digital signal processor 116 that a valid target hasbeen detected, processing is begun to determine the identity (ID), depthand size of the target. This is described in reference to FIGS. 11, 12,13, 14, 15, 16 and flow diagram FIGS. 17C, D and E. Referring now toFIG. 17C, in operational block 220 data is collected for another tensamples. Referring to FIG. 11, there is shown a 64 point buffer and aline 221. The ten additional samples moves the buffer forward to line221 and essentially captures the entirety of the pulse from the target.

Moving to operational block 222, an examination is made to determinewhether the high or low gain should be used for the S1 and S4 signalsthat are stored in the buffers. If any of the high gain signals aresaturated, then the low gain signals are used. Otherwise, the high gainamplitude samples are used.

Moving to operational block 224, the S1 and S4 data are copied to FFTbuffers as shown in FIG. 12. The peak of the target signal is aligned atthe center of the buffer. From the lower right edge of the target signalto the right side of the buffer, a linear approximation of the signal ismade.

Moving from operational block 224 to question block 226, an inquiry ismade to determine if the S1 signal in the FFT buffer has any multipletargets. Referring to FIG. 12, the two smaller signal pulses to the leftconstitute such multiple targets. If no such targets exist, the no exitis taken from question block 226 and entry is made to the bubble C. Ifsuch targets are present, entry is made to the operational block 228 inwhich these other targets are erased from the FFT buffer. The removedtarget signals are replaced with a fixed value so that the target signalis essentially symmetric in the FFT buffer.

The S4 signal in a similar fashion is transferred to an FFT buffer,centered and the extraneous signals extracted so that the S4 signal isready for Fourier transform processing in the same manner as the S1signal shown in FIG. 12.

Referring now to FIG. 17D, a calculation is now performed to produce theFourier transforms for the S1 and S4 signals as shown in operationalblock 230. Referring to FIG. 13, there is shown the direct Fouriertransform of the S1 signal. A similar transform is made for the S4signal. This signal, however, includes both the real and the imaginarycomponents of the Fourier transform. Because of the symmetry of thetarget signal in the FFT buffer (FIG. 11), the signal component ofinterest is the real component. The real component of the Fouriertransform shown in FIG. 13 is shown in FIG. 14. This comprises discreteinteger frequencies and the data points in FIG. 14 are marked withcircles. The data points 233A, B, C, D and E represent odd frequencies1, 3, 5, 7 and 9. The data points 233F, 233G, 233H and 2331 representrespectively the even integer frequencies 2, 4, 6 and 8. Note also thatthe positive data points are along an envelope 235 and the even datapoints are along an envelope 237. These envelopes are monotonicallydecreasing toward zero.

Referring now to operational block 232 in FIG. 17D, an examination ismade for the Fourier transform for the S1 signal as shown in FIG. 14 andfor the corresponding S4 signal, which has been processed in the samemanner. Only the frequencies in the range of three to fifteen hertz, forthe selected embodiment, are examined. Frequencies above fifteen hertzare likely noise, such as strikes of the metal detector against anobject and frequency components below three hertz are likely due toground or mineralized soil. A further test is made to determine overwhat range there is a monotonically decreasing envelope as shown in FIG.14. A further test is to determine that the odd integer frequencies arepositive and the even integer frequencies are negative. This is thecondition shown in FIG. 14.

Moving now to operational block 234, an examination is made to determinethe maximum frequency range which fits the pattern, that is, therestrictions set forth in operational block 232. This is indicated bythe bidirectional arrow which indicates a frequency range 239 and thisis a range of approximately three to nine hertz.

FIG. 15 shows a transform signal S1 which does not meet the requiredcharacteristics. FIG. 16 shows a transform signal S1 which does have afrequency band which meets the requirements as shown in FIG. 14.

Moving now to operational block 236, shown in FIG. 17D, the energy forthe S1 signal in the selected frequency range, shown in FIG. 14, issummed. The frequency range determine for the S1 signal, this is,frequency range 239, is arbitrarily applied to the S4, signal which hasbeen processed in the same manner to produce a similar Fourier transformas shown in FIG. 14. The energy of the integer components in the S4signal are likewise summed for the frequency range 239.

Operational control is now transferred to block 238 in which the sum ofthe S4 Fourier components is divided by the sum of the S1 Fouriercomponents to produce a ratio.

Moving from operational block 238 to operational block 240, this ratiois scaled according to the mix of high and low gain signals which wereused. The high or low scale factor must be taken into account to producethe correct numerical ratio. Control is then transferred through bubbleD to the start of operation shown in FIG. 17E.

In operational block 250, the DSP 116 looks up in an ID table to findthe ID number derived from the ratio of the S4 and S1 signals determinedin operational block 240. This produces an ID number, that is, anidentification for the particular electrical characteristics of thetarget material.

In operational block 252, the ID number is transferred to themicroprocessor for display on the display screen 42. This can indicate,for example, that the target is a coin, pull-tab or other particulartype of object.

Transferring now to operational block 254, the DSP 116 produces an audioresponse appropriate for the identified target and passes this to themicroprocessor which in turn causes the audio circuit 122 to produce theparticular audio response, such as indicating detection of a desiredtarget or perhaps indicating detection of an undesired target. Further,this can generate a speech signal which can verbally identify theparticular target. Circuit 122 produces no audio for an undesiredtarget.

Upon completion of the operations in block 254, control is returned tothe start in FIG. 17A.

previously in reference to FIG. 17B, when the trigger threshold had beenachieved, that is, identification of a target signal had been made inquestion block 196, control is transferred to operational block 198 tocalculate the depth and size of the target. This operation is describedin reference to FIG. 17F. Entry is first made to an operational block280. Within this block, the buffers of information for signal S1 and S5are examined to determine if any of the high amplitude signals are insaturation. If they are in saturation, the low gain signals are used.

In operational block 282, the difference between the minimum and maximumamplitudes of the target signal, such as shown in FIG. 11, is determinedfor the S1 signal. Referring to FIG. 11, there are two "x" marks whichindicate the minimum and maximum values of this signal.

Transferring to operational block 284, a similar determination is madefor the signal S5 at the same points in time.

Transferring to operational block 286, the differential signaldetermined for the S5 signal is divided by the differential signal (peakto peak) for the S1 signal.

Transferring from operational block 286 to operational block 288, thisratio, produced in block 286, is scaled depending upon the high and lowgain signals which were used.

Transferring to operational block 290, reference is made to a depthtable based on the ratio to determine one of the six possible depths asindicated in FIG. 2 for the display 42. A lower ratio indicates thedeeper the target is in the earth.

From block 290, control is transferred to operational block 292 in whichthe determined depth, the ID (characteristic) of the target and thesignal strength of the target, that is, the amplitude of S5 signal, istransferred to the microprocessor 120 for using the look-up table inFIG. 6 for determining the size of the target. Control is then returnedto the start of the DSP processing in FIG. 17A. FIG. 6 gives size as afunction of depth and amplitude.

Optionally, in place of using S5 alone as the amplitude signal, acombination of S5 and S1 may be used. This can be a linear combinationsuch as a sum. A further option is S1 alone.

Operation of the microprocessor 120 is described now in reference toFIGS. 18A and 18B. In operational block 300, upon powering up the metaldetector 30, the microprocessor initializes the hardware in aconventional fashion.

In the next operational block 302, the microprocessor 120 scans thekeypad 44 to detect inputs from the operator. This operation isrepeatedly done to determine when the operator makes changes to theoperating configuration for the metal detector.

In operational block 304, the inputs from the keyboard are processed andany changes to modes or controls for a digital potentiometers are madeaccording to the keys which have been pressed.

In operational block 306, the modes selected by the user and settingsmade by the user are transferred to the DSP 116 for its use, asdescribed above, in processing signal information.

In operational block 308, the microprocessor 120 receives the IDcalculation made by the digital signal processor, the depth calculationand the measurement of the signal magnitude which all have been made bythe DSP 116. By use of this information and reference to the look-uptable 150 shown in FIG. 6, which is stored in the microprocessor 120, adetermination of the size classification from A through E is made.

In operational block 310, signals are sent by the microprocessor 120 toactivate the liquid crystal display 42 for displaying modes and usersettings, which have been selected previously by the user, fordisplaying the ID of the target which has been determined by the digitalsignal processor 116, the depth which has been determined by theprocessor 116 and the size which has been determined by themicroprocessor 120 in reference to table 150.

In operational block 312, in FIG. 18B, the audio signal identifierreceived from the digital signal processor 116 is sent to the audiocircuit 122 provided that the other information received from thedigital signal processor meets the mode and ID requirements which havebeen set by the user.

In operational block 314, the audio signal is modified according to thethreshold and volume settings provided by the operator and then used todrive a speaker.

In operational block 316, a speech chip is activated as needed toproduce a speech output if it has been selected by the user, asindicated by the functions available to the operator of the metaldetector 30, as shown in FIG. 2. Upon completion of operational block316, control is transferred back to operational block 302 to againresume scanning the keypad 44 for operator inputs and then sequentiallygo through the following blocks as described.

The selected embodiment described for the present invention is aninductively balanced detector, but many aspects of the present inventioncan be used with a pulse detector as well.

Although one embodiment of the invention has been illustrated in theaccompanying drawings and described in the foregoing detaileddescription, it will be understood that the invention is not limited tothe embodiment disclosed, but is capable of numerous rearrangements,modifications and substitutions without departing from the scope of theinvention.

What we claim is:
 1. A method for determining a size classification foran object detected by a metal detector, comprising the stepsof:transmitting a signal from a transmit coil which is spaced proximatefirst and second receive coils wherein said object causes respectivesignals to be produced in said first and second receive coils when saidobject is in the vicinity of said transmit and receive coils, andwherein said first receive coil is spatially offset from said secondreceive coil, detecting a first signal from said first receive coil,detecting a second signal from said second receive coil, determining aratio of said first signal and said second signal, and selecting one ofa plurality of predetermined size classifications for said object by useof a predetermined table which relates each of said size classificationsto factors including at least the amplitude of said first signal andsaid ratio of said first signal and said second signal.
 2. A method fordetermining a size classification for an object as recited in claim 1wherein the ratio of said first signal and said second signal isproportional to an approximate distance between said coils and saidobject.
 3. A method for determining a size classification for an objectas recited in claim 1 wherein said first and second signals areessentially phase quadrature components of the respective receivesignals with respect to the signal in said transmit coil.
 4. A methodfor determining a size classification for an object as recited in claim1 wherein said ratio comprises said second signal divided by said firstsignal.
 5. A method for determining a size classification for an objectas recited in claim 1 wherein said first and second receive coils arecoaxial.
 6. A method for determining a size classification for an objectas recited in claim 1 wherein said first receive coil is larger thansaid second receive coil and said first and second receive coils arecoaxial.
 7. A method for determining a size classification for an objectas recited in claim 1 wherein said first and second receive coils aresubstantially inductively balanced with said transmit coil.
 8. A methodfor determining a size classification for an object detected by a metaldetector, comprising the steps of:detecting a first signal from saidfirst receive coil, detecting a second signal from said second receivecoil, determining a ratio of said first signal and said second signal,and selecting one of a plurality of predetermined size classificationsfor said object by use of a predetermined table which relates each ofsaid size classifications to the combination of the amplitude of acombination of said first signal and said second signal and said ratioof said first signal and said second signal.
 9. A method for determininga size classification for an object as recited in claim 8 wherein theratio of said first signal and said second signal is proportional to anapproximate distance between said coils and said object.
 10. A methodfor determining a size classification for an object as recited in claim8 wherein said first and second signals are essentially phase quadraturecomponents of the respective receive signals with respect to the signalin said transmit coil.
 11. A method for determining a sizeclassification for an object as recited in claim 8 wherein said ratiocomprises said second signal divided by said first signal.
 12. A methodfor determining a size classification for an object as recited in claim8 wherein said first and second receive coils are coaxial.
 13. A methodfor determining a size classification for an object as recited in claim8 wherein said first receive coil is larger than said second receivecoil and said first and second receive coils are coaxial.
 14. A methodfor determining a size classification for an object as recited in claim8 wherein said first and second receive coils are substantiallyinductively balanced with said transmit coil.
 15. A method fordetermining a size classification for an object as recited in claim 1wherein there are five of said size classifications.
 16. A method fordetermining a size classification for an object as recited in claim 8wherein there are five of said size classifications.